There have been developed a variety of phosphors for use in PDP and VFD, including ZnO:Zn (using ZnO as a matrix material and Zn as a luminescence center) as the phosphor capable of blue green emission, ZnS:Ag and ZnS:Cu as the blue phosphor, and (ZnCd)S:Ag+In2O3 as the red phosphor. Research works continue on many other phosphor materials. Of these, the phosphors capable of blue green emission have already been used in commercial PDP and VFD because light emission of a high luminance is possible at a relatively low drive voltage.
As to the blue and red phosphors, however, the emission luminance is insufficient. It is desired to develop blue and red phosphors which produce light emission of high color purity at a low power consumption.
In the recent years, active research works have been made on thin-film EL devices as small or large-size, lightweight flat panel displays. A monochromatic thin-film EL display using a phosphor thin film of manganese-doped zinc sulfide capable of emitting yellowish orange light has already become commercially practical as a double insulation structure using thin-film insulating layers 2 and 4 as shown in FIG. 3. In FIG. 3, a predetermined pattern of lower electrodes 5 is formed on a substrate 1, and a first insulating layer 2 is formed on the lower electrode-bearing substrate 1. On the first insulating layer 2, a light-emitting layer 3 and a second insulating layer 4 are successively formed. On the second insulating layer 4, a predetermined pattern of upper electrodes 6 is formed so as to construct a matrix circuit with the lower electrodes 5. As a general rule, the phosphor thin film is annealed at temperatures below the strain point of the glass substrate in order to enhance luminance.
More recently proposed was a structure using a ceramic substrate as the substrate 1 and a thick-film dielectric layer as the insulating layer 2. Another device structure was proposed in which a high permittivity BaTiO3 thin plate is used as the substrate and an electrode is formed on the back of the substrate so that the thin plate serves as an insulating layer and substrate. Since ceramics such as alumina and BaTiO3 are used as the substrate, these structures permit the phosphor thin film to be annealed at high temperatures for providing an increased luminance. Also, since a thick film or thin plate dielectric layer is used as the insulating layer, these structures are resistant to insulation breakdown as compared with EL devices using a thin film as the insulating layer. Advantageously, more reliable panels can be manufactured. Then a structure of sandwiching a phosphor thin film like the double insulation structure is not necessarily needed. The insulating layer may be a single thick film or thin plate dielectric layer only on one side.
Thin-film EL displays must display images in color in order that they find use as computer, TV and similar monitors. Thin-film EL displays using sulfide phosphor thin films are fully reliable and resistant to environment, but at present regarded unsuitable as color displays because EL phosphors required to emit light in the primary colors of red, green and blue have poor characteristics. Engineers continued research on SrS:Ce (using SrS as a matrix material and Ce as a luminescence center) and ZnS:Tm as a candidate for the blue light-emitting phosphor, ZnS:Sm and CaS:Eu as a candidate for the red light-emitting phosphor, and ZnS:Tb and CaS:Ce as a candidate for the green light-emitting phosphor.
These phosphor thin films capable of emitting light in the primary colors of red, green and blue suffer from problems of emission luminance, emission efficiency and color purity. Thus color EL panels have not reached the commercial stage. With respect to red, in particular, it is known that CaS:Eu produces light emission of relatively good color purity. Improved phosphors are disclosed in JP-A 1-206594 and JP-A 2-148688. Their emission factors including luminance and efficiency are still short as the red color for full-color display. As described in JP-A 2-51891 and TV Society Technical Report Vol. 16, No. 76, pp. 7-11, the response time is as long as several seconds to several tens of seconds. These phosphors as such are impractical as the red light for a full-color moving image display which must make real-time response to drive signals.
With respect to the red color, a customary practice for acquiring red light is to use a ZnS:Mn film which is an orange phosphor thin film having a high luminance and efficiency and pass the light through a color filter to cut out red light in the wavelength region necessary as the panel from the EL spectrum of the phosphor thin film. Use of a filter complicates the manufacture process and, still worse, brings about a lowering of luminance. When red is taken out through the filter, the luminance of red EL phosphor thin film suffers a loss of 10 to 20% so that the luminance is reduced below the practically acceptable level.
To simultaneously solve the above-discussed problems associated with the phosphors for use in light emitting devices including EL devices, PDP and VFD, there remains a need for a red phosphor material capable of emitting light of a sufficient color purity to eliminate a need for a filter, at a high luminance and in good response.